NAIROBI, Kenya (AP) – Kenya’s interior minister says he has suspended nine security chiefs in the eastern town of Garissa during a probe to determine if they failed to stop an extremist attack earlier this month in which 148 people died.

Joseph Nkaissery said a thorough investigation is needed to determine if the individual officers are criminally culpable.

Nkaissery and the Police Inspector General Wilson Boinnet have come under heavy criticism for flying to Garissa University College, the scene of the April 2 attack, instead giving those flights to members of the police tactical team that ended the siege hours later.

Al-Shabab extremists from Somalia claimed responsibility for the university attack. Al-Shabab has vowed to carry out attacks on Kenya for deploying troops to fight the extremist group.